1. Field of the Invention
This invention relates to semiconductor substrate processing. More particularly, this invention relates to apparatus and method for rotationally aligning and degassing a semiconductor substrate in the same vacuum chamber.
2. Description of the Related Art
In the processing of semiconductor substrates or wafers in the formation of integrated circuit structures thereon, it is important that the wafer be thoroughly degassed to remove adsorbed gases, moisture, etc. from the wafer prior to, for example, performing a physical vapor deposition (PVD) process to deposit materials on the wafer by sputtering from a target in a vacuum processing chamber. Other processes such as advanced chemical vapor deposition (CVD) processing, may also require degassing of the wafer. Degassing prior to PVD processing conventionally is carried out at temperatures exceeding 350.degree. C. for time periods of from about 40 seconds to about 2 minutes to remove sufficient gases from the wafer to assure a satisfactory deposition by sputtering. Outgassing of substrates during aluminum PVD is more severe than during the prior CVD steps because the PVD process is performed at much higher vacuums and somewhat higher substrate temperatures, both of which induce greater outgassing. Therefore, to avoid outgassing from contaminating the PVD process, the de-gassing of the wafer before the first PVD step must be more extensive than the de-gassing performed before the CVD steps.
Degassing of a wafer is conventionally carried out in one of two ways. One method used to degas a wafer comprises a radiant heating of the wafer, using heat lamps located external to the vacuum chamber containing the wafer, and positioned adjacent transparent windows through which the heat is radiated from the lamps to the wafer. This method is relatively low in cost, is fairly rapid, and does not require clamping the wafer to the wafer support within the vacuum chamber. However, the radiant heating method is unsatisfactory for temperatures in excess of 350.degree. C., because the temperature of the wafer is not easily controlled, and the heating is usually not uniform across the entire wafer. Typical temperature nonuniformity across the wafer at 350.degree. C. is greater than .+-.30.degree. C. Furthermore, alignment of the rotational orientation of the wafer, during the degassing step, is usually not possible because the radiation from the heat lamps interferes with operation of the optical means conventionally used for such rotational alignment.
The other method conventionally used to degas a wafer, particularly when subsequent PVD processing will be carried out which requires degassing at temperatures in excess of about 350.degree. C., comprises physically (mechanically) clamping the wafer to a wafer support in a vacuum chamber and then heating the wafer using a resistive heater located in the wafer support adjacent the undersurface of the wafer resting on the wafer support. However, since the wafer normally only physically touches the wafer support at the physically clamped periphery or edges of the wafer, and the transmission of heat from the heater in the wafer support to the underside of the wafer via conduction through a vacuum is very poor, a thermally-conductive gas is normally admitted into the space between the wafer support and the underside of the wafer, with the clamped edge of the wafer serving to at least partially retain the gas in this space. This heating method permits degassification temperatures of as high as about 500-600.degree. C. to be achieved.
This method thus permits the use of degassing temperatures in excess of 350.degree. C., and permits measurement and reasonable control of the temperature of the wafer. However, alignment of the rotational orientation usually cannot be carried out during the degassing step because the conduit for the thermally conductive gas inhibits rotation of the chuck. The clamping ring also inhibits rotation due to its weight. Rotation of a clamped wafer could also cause wafer breakage and particles. The alignment of the rotational orientation of the wafer must, therefore, be carried out in a separate chamber prior to the degassing step. Furthermore, because this form of degassing must be preformed in a chamber very similar to a PVD chamber (i.e., it must include a cryopump, heated chuck, wafer lift assembly, cryo isolation valve, transfer chamber, isolation valve, clamp ring, etc.), it is a very expensive solution. Also typical temperature uniformities across the wafer achieved with this type of degassing apparatus are approximately .+-.10 to 15.degree. C. Temperature uniformities of .+-.5.degree. C. are required for advanced devices.
Furthermore, regardless of which heating method is used, because of the extended time period needed for degassing prior to PVD processing, the degassing step can reduce process throughput. One prior art approach which has been considered for solving this particular problem is to provide parallel degassing chambers, i.e., two degassing chambers are provided in a semiconductor wafer processing apparatus for each PVD processing chamber. However, this adds considerable extra cost to the apparatus. In addition, when the rotational orientation of the wafer must also be carried out in a separate chamber, either three or four preprocessing chambers must be utilized (depending whether or not each of the two parallel degassing chamber is coupled to its own separate rotational orientation chamber), which greatly adds to the overall expense of the apparatus.
It would, therefore, be desirable to be able to consolidate the rotational alignment and degassing of the wafer into a single chamber which would avoid the expense of separate chambers, as well as the additional time consumed during transfer of the wafer from one chamber to the other. It would be of further advantage if the degassing could be carried out at high temperatures, i.e., temperatures in excess of about 350.degree. C., without mechanically clamping the wafer to the wafer support, and while still maintaining an even and controllable heating of the wafer. It would be even more advantageous if both the degassing and the rotational orientation of the wafer could be carried out simultaneously in the same chamber at a high temperature and without mechanical clamping the wafer to the wafer support.